Electron tube structure



Jan. 27, 1959 o. H. SCHADEV ELECTRON TUBE STRUCTURE Filed Sept. '15,1955 `I N V EN TOR. 07m Saw/:of

: Mam/L nited ELECTRON TUBE STRUCTUREl Otto H. Schade, West Caldwell, N.J., assignor to Radio Corporation of America, a corporation of DelawareApplication September 15, 1955, Serial No. 534,448

4 Claims. (Cl. 313-238) This invention relates to an electron tubestructure of voltage peak, that is, when the anode is negative withrespect to the cathode, the anode temporarily becomes a cathode. Whenthe electrostatic field density between the anode and cathodel isrelatively high, ofthe order of say 100,000 volts per centimeter (suchas when the tube is operated at inverse peak voltages of over, say30,000 volts), cold or field emission takes place. Electrons are pulledfrom the anode and are accelerated toward the cathode and toward metalmembers on which the cathode is supported. Some of the electrons missthe metal members and bombard the envelope. This electron bombardmenteventually results in a rupture of the envelope. In previous rectifiertubes, such as tubes of the type wherein the cathode is supported withina tubular anode by a pair of parallel wires, the relatively smalldiameter of the wires gives rise to relatively high electrostatic fielddensities. Some of the electrons from the anode are attracted into thespace between the cathode support wires (the support wires acting as agrid) and are accelerated by the electrostatic field between the wiresto a portion of the glass envelope resulting in increased bombardment ofthe envelope.

Accordingly, it is an object of the invention to provide an improvedrectifier tube structure which is economical of manufacture and whereinthe destruction of the envelope, by electron bombardment from the anodeof the tube, is substantially eliminated.

in high voltage rectifier tubes and electrostatic stresses between theanode and cathode often result in the cathode being torn from itssupport. The cathode is usually mounted in the center of a tubularanode. 1f the cathode is moved out of this center position during tubeoperation, such as when the tube is subjected to mechanical shock, theelectrostatic stresses on opposite sides of the cathode are no longerbalanced and the cathode is attracted by the anode with a force suchthat it is physically moved to the anode. In order to minimizeelectrostatic forces on the cathode, previous tubes have resorted to theuse of hemispherical anodes and bow-shaped cathodes with the attendantincreased cost and complexity of the tube. The bow-shaped cathode wassubjected to relatively high stresses since the electrostatic forceurging it vto the anode was directed to only one side of the cathode.The use of a hemispherical, as distinguished from a tubular, anode alsoresulted in an exposure of a relatively large area of the tube envelopeto electron lbombardment by the aforementioned field emission from theanode.

It is a further object of the invention to provide an Patentimprovedrectifier tube having a structure in which the cathode isv maintainedsubstantially free of unbalanced and a tubular cathode support incoaxial relation andr with the cathode support having a portion thereofterminating an appreciable distance within the anode. A cathode ismounted on the end of the cathode support within the anode. The coaxialanode and cathode support arrangement provides a relatively strongmechanical support for the cathode; this better enables the cathode towithstand momentary unbalanced electrostatic stresses such as thosewhich would be caused by mechanical shock during tube operation. In oneembodiment of the invention, the tubular cathode support is providedwith a skirt which has a radial extent from the tube axis at least asgreat as that of the inside extent of the anode and which is axiallyspaced a distance from the anode about twice that between the anode andthe cathode support. The cathode support and the anode are preferably intheform of cylinders having a ratio between their diameters of the orderof two to one for providing a minimum electrostatic field concentrationat any point in the space within the anode. The inner, cathode supportcylinder physically intercepts most of the electrons from the anodeduringl the periods of inversevoltage. The electrons which escape fromthe space within the anode are intercepted by the skirt. Theelectrostatic held, between the anode and the tubular portion and skirtof the cathode support, focuses these electrons onto the skirt. Thus thetube envelope is preserved from electron bombardment during periods ofinverse voltage.

In the sole figure of the drawing there is illustrated a high voltagerectifier tube of thetype similar to that known commercially as the 3B2.The tube comprises a glass envelope 1 having at one end thereof a base2. Within the envelope referred to are mounted a tubular anode 3 and acathode 4. The anode 3 is closed at one end 5. This end 5 is fixed to atubular access terminal 6 for supporting the anode 3 and for providingelectrical access thereto. The other end 7 of the anode 3 is open.Adjacent to its open end' 7, the anode 3 has outwardly ared edges 8 toprevent high electrostatic field concentration at this end. The cathode4 is supported at one end thereof by a Wire 9 which is positioned alongthe axis of the tube andl at the other end thereof by a second wire 10which is fixed to a relatively heavy, generally U-shaped, wire loop 11.The wire loop 11 is in turn fixed at -its ends to a cathode supportcylinder llZ which is concentric with the anode 3. The wire loop 11 actsas a low mu grid; it lowers the space potential adjacent to the cathode4 since the loop is electrically connected to the cathode. This reducesthe magnitude of the electrostatic forces of the attraction between thecathode 4 and the anode 3 acting directly on the cathode. The cathodesupport cylinder is supported by a metallic shield or skirt 13 whichprotects fromkelectron bombardment the portion of the glass envelope 1adjacent to the base 2. The skirt 13 has a radial extent from the tubeaxis at least as great as that of the inside radial extent of the anode3- and is axially spaced a distance from the anode about twice thatbetween the cathode support and the anode. The tubular cathode support12 and the central wire 9 are connected to appropriate ones of prongs 14mounted on thebase 2. The cathode 4 is operated at a predeterminedreference po-v tential, and the anode 3 is adapted to have analternating current potential applied to it. In operation, the potentialdifference between the anode and cathode are Patented Jan. 27, 1959often of the order of 70,000 volts in the tubes of the aforementionedtype 3B2.

The coaxial arrangement of the cathode support and anode provides arelatively strong support for the cathodel 4 to better enable it towithstand mechanical shock during tube operation Without being torn fromits sunport by unbalanced electrostatic stresses. Since the cathodesupport is in the form of a cylinder, the free or unsupported end of itdoes not bend appreciably even when the tube is subjected to shock. Thusthe cathode is maintained along the anode axis where the radialelectrostatic forces on the cathode are equal in all directions,

The anode cylinder 3 and cathode support cylinder 12 have a ratiobetween their diameters of the order of about two to one in order to atthe same time (a) prevent bombardment of the envelope by electrons fromthe anoder during periods of inverse voltage and (b) avoid fieldemission from the anode so that no appreciable inverse current flow(that is, current from the 'anode to the cathode) occurs within thetube. As the diameter of the cathode support cylinder is decreased theelectrostatic field density about any place on the surface of thiscylinder is increased. This is so because, as is known, for twoconcentric cylinders the eld density around the inner cylinder increaseswith decreasing inner cylinder diameter. Consequently, in order toprovide a minimum electrostatic field density it would seein desirableto use as large a support cylinder diameter as could be mechanicallyaccommodated within the tube. A large diameter support cylinder is alsodesirable since greater numbers of electrons, traveling from the anodeto the cathode, are then intercepted by the support cylinder. Then, too,the cathode is provided with a greater mechanical stability with anincreased support cylinder diameter. However, as the cathode supportcylinder diameter is increased, and the anode cylinder diameter remainsconstant, the space between the anode and the support cylinder isdecreased; increased field emission from the anode to the supportcylinder then takes place due to the close adjacency of the two members.A condition is soon reached in which the field emission is so great thatappreciable inverse current flow occurs within the tube, and the tubeceases to act as a rectifier since it passes current in both directions.Thus, a compromise in the ratio of anode diametei' to support cylinderdiameter must be had in order to eiiect an optimum relation betweenelectrostatic field density and field emission. This optimum relationexists in a ratio between the inside diameter of the anode and theoutside diameter of the cathode support cylinder ot the order of about 2to l, that is, when the `outside diameter of the support cylinder isbetween 40 percent and 60 percent of the inside diameter of the anode.For example, in a tube of the aforedescribed type operated at 70,000volts peak-to-peak voltage and having an anode cylinder with an insidediameter of 1%6 inch and a cathode support cylinder with an outsidediameter of 1%2 inch, an appreciable direct current output was obtainedwithout inverse current ow and without electron bombardment of theenvelope. Ori the other hand, when the same lfyf; inch inside diameteranode was used with a 1%@ inch diameter support cylinder, the envelopeglowed under electron bombardment; and when the same anode was used witha 1/2 inch diameter support cylinder, no appreciable direct currentoutput was obtained due to high field emission from the anode to thecathode. The actual sizes of the support cylinder and the anode may bescaled up or down for correspondingly higher or lower voltage operationas long as a .ratio between the diameters is of the order of 2 to 1.

To further insure that electrons emitted from the anode cylinder 3during periods of inverse voltage are prevented from bombarding theenvelope, the configuration of the anode cylinder and rcathode supportcylinder 12 structure is chosen such that while, optically, one can seethe inside of the anode from the glass envelope, yet electrically,electrons from the inside of the anode cannot see the envelope due tofrefraction or deflection of the electrons by the space potential whichis indicated by equipotential lines 15. The relatively few electronswhich escape from the space between the anode and the cylindricalportion of the cathode support are intercepted by the skirt t3.Interception of the electrons by the skirt is effected by virtue of itsrelatively large radial extent and by its relatively close axial spacingfrom the ancde. 'the electrostatic lield established in the spacebetween the anode and the skirt defiects to the skirt electrons(indicated by line 16) which would otherwise have a path ot travelthrough the space between the anode and skirt and on to the envelope.While it is 'desirable to have the anand skirt as close to each other aspossible in order to establish as dense an electrostatic detiectingfield as possible, the anode and skirt must be spaced far enough fromeach other to prevent appreciable iield emission from taking placebetween these two members and to prevent excessive dielectric stress ofthe glass envelope, the latter occurring when adjacent portions of thespace adjacent to the envelope are maintained at high potentialdifferences. Since the spacing between the anode cylinder 3 and thecathode support cylinder 12 represents the closest practical spacingbetween members for the voltages at which the tube is adapted tooperate, the spacing between the anode and skirt is at least as great asthat between the cylindrical members 3 and l2. Actually, since there-entrant edge S of the open end 7 of the anode has a relatively smallradius of curvature, the axial spacing between the anode and skirt ispreferably greater than that between the two cylinders in order toinsure that no appreciable field emission takes place. A spacing betweenthe anode and skirt of about twice that between the cylindrical members3 and 12 has proven optimum.

From the foregoing it will be apparent that the invention provides animproved rectier tube wherein the envelope is adapted to be maintainedsubstantially free of electron bombardment, and the cathode is adaptedto be maintained substantially free of electrostatic stresses eventhough subjected to mechanical shock.

What is claimed is: l

l. An elongated rectilier tube comprising an envelope containing ahollow cylindrical anode closed at one end only; an elongated cathodemounted within said anode', and a support structure supporting saidcathode within said anode; said support structure including a hollowcylindrical member coaxial with said anode and extending 'into the openend of said anode for a distance equal to about half of 'the insideaxial extent of said anode, a U-shaped metallic loop fixed at the endsthereof to the end of said cylindrical member within said anode andextending toward the closed end of said anode, andl a wire disposedcoaxially within said cylindrical member and extending beyond the end ofsaid member within said anode and spaced from said loop; said cathodebeing iixed at one end thereof to the end of said wire extending beyondsaid member and at the other end thereof to a portion of said loopintermediate said loop ends; whereby said loop is adapted to reduce,during tube operation, the magnitude of the electrostatic force ofattraction between said anode and said cathode and acting directly onsaid cathode.

2. A rectifier tube including an envelope containing a hollowcylindrical anode closed at one end; a cathode mount comprising a hollowcylindrical member, a U- shaped metallic loop tixed at the ends thereofto one end of said cylindrical member and extending away from saidmember, a wire having a portion thereof disposed coaxially within saidcylindrical member and extending beyond said one end of said member andspaced from said loop, and a cathode iixed at one end thereof to the endof said wire portion extending beyond said member and at the other endthereof to a portion of said loop intermediate said loop ends; wherebysaid loop is adapted to reduce,

during tube operation, the magnitude of the electrostatic force ofattraction between said anode and said cathode and acting directly onsaid cathode.

3. A rectifier tube comprising an envelope containing a hollowcylindrical anode closed at one end, a hollow cylindrically tubularcathode support having a common axis with said anode and extending intothe other end of said anode for a distance equal to about half of theinside axial extent of said anode, the outside diameter of said cathodesupport being between 40 percent and 60 percent of the inside diameterof said anode, a metallic skirt xed to said cathode support and having aradial extent from the common anode and cathode support axis at least asgreat as that of the radial extent of said anode and spaced axially adistance from said other end of said anode about twice the radialdistance between said anode and said cathode support, and a cathodemounted on the end of said cathode support within said anode.

4. A rectifier tube including an envelope containing a hollowcylindrical anode closed at one end; a cathode mount comprising a hollowcylindrical member extending concentrically to within said anode, a rstcathode connector and support means fixed to the end of said hollowcylindrical member within said anode and extending away from said membertoward the said closed end of said anode, a second cathode connector andsupport means comprising a wire having a portion thereof disposedcoaxially Within said cylindrical member and extending beyond the saidend of said member within said anode, and a cathode connected andsupported between said irst and said second cathode connector andsupport means.

References Cited in the le of this patent UNITED STATES PATENTS1,647,238 Manthorne Nov. 1, 1927 2,332,428 Atlee et al Oct. 19, 19432,397,982 Salzberg Apr. 9, 1946 2,656,479 Brown Oct. 20, 1953 2,719,935Muller Oct. 4, 1955

